In a typical prior art stretching method, such as a SIGNODE.RTM. process, a cast sheet of thermoplastic material, for example, polypropylene, is first reduced in size by rolling it through a pair of closely spaced milling rollers or cylinders that rotate in opposite directions. After the thickness of the sheet is reduced, the sheet is drawn and stretched out of the milling rollers by a series of orienting rollers or a bridle assembly to its final desired size.
Another prior art method that is commonly used is a process called the short gap method and is generally comprised of an entry bridle, a stretching assembly and an exit bridle. A slow speed, heated bridle assembly advances a cast sheet of material, usually film, to a stretching assembly. The stretching assembly is comprised of a pair of rollers or cylinders set a distance apart. The first roller rotates at the same speed as the entry bridle. The second roller is rotating faster than the first roller and at the same speed as the exit bridle. Thus, as the film passes through the assembly, it is stretched to its final desired size.
These prior art methods present several disadvantages. The properties of the straps produced by these methods provide limited increases in strength without significant decreases in other desired properties. Also, substantial necking occurs as the sheet is stretched over the distance between the rollers.
The present invention presents a novel milling and stretching apparatus and method intended to minimize these problems, as well as to present several other improvements.